This invention relates to the reduction of aerodynamic drag on vehicles such as tractor-trailers, as well as other objects that experience relative motion with respect to surrounding fluid.
It is well-known that aerodynamic drag on vehicle bodies has a number of undesirable effects, including a reduction in fuel economy. These effects are particularly acute in the case of semi-trailers and intermodal containers, which have a box-like shape. In addition to environmental concerns, rising fuel prices contribute to increases in shipping costs that must be passed along to the public.
There have been efforts in the prior art to provide drag reducing devices for semi-trailers and other vehicles. For example, U.S. Pat. No. 2,737,411 discloses an inflatable streamlining apparatus for vehicle bodies. Some aerodynamic drag reduction devices are permanent and do not change configuration with changes in vehicle speed. These include side panels such as U.S. Pat. Nos. 4,451,074, 4,518,188, and 7,740,303, and other airflow shaping devices such as U.S. Pat. Nos. 3,960,402, 3,999,797, 5,280,990, 6,986,544.
Other aerodynamic drag reduction devices described in the prior art do change configuration. These devices can be divided into two basic categories: rigid and flexible. Rigid devices use plates and/or shaped panels that change configuration using hinges or other hardware, such as U.S. Pat. Nos. 4,257,641, 4,458,936, 4,508,380, 4,682,808, 5,348,366, 6,092,861, and 7,854,468. Flexible devices described in the prior art are inflated, such as U.S. Pat. Nos. 4,006,932, 4,601,508, 4,741,569, 4,978,162, 5,236,347, 5,375,903, and 7,866,734.
Rigid structures can maintain their shape against air currents, but have the disadvantage of adding weight and complexity to the loading/unloading process (e.g., the panels must be swung to the side to open the trailer). Rigid structures are also easily bent or otherwise damaged (e.g., rigid side skirts are often deformed if the trailer is pulled across a mound such as railroad tracks). In addition, devices that create the greatest reduction in aerodynamic drag require complex surface curves, which are expensive to manufacture, so most such devices use less efficient flat surfaces.
Devices such as air-channeling vanes cause fewer problems with loading/unloading, but are also easily deformed, which causes a loss of effectiveness, and the overall reduction in aerodynamic drag is much less than is possible with devices that change the shape of the vehicle.
Flexible devices solve some problems inherent in rigid structures, and can be made more effective in reduction of aerodynamic drag, but present certain different problems. In particular, the prior art uses airtight bags and/or tubes that depend on a relative positive pressure compared to the environment to maintain the desired shape. Devices that utilize air scoops to inflate and create that positive pressure have difficulty maintaining shape unless the air scoops are large and located directly in the air stream, thus creating new aerodynamic drag. Other devices of the prior art utilize a positive pressure pumping system to inflate, but this adds complexity, requires driver input, and often delays opening the trailer.
U.S. Pat. No. 7,740,304 discloses a device that utilizes a flexible skin over hinged ribs similar in concept to an umbrella. In particular, the “flexible skin material” is attached to both the frame and the trailer body. A complex arrangement using levers, cables, pulleys, and a “scissor jack mechanism” is required to deploy and stow the device.
The present invention recognizes the foregoing considerations, and others, of the prior art.